Fig. 10.3). In humans it is located on the parts of the precentral gyrus that form the rostral wall of the central sulcus and the caudal half of the apex of the gyrus. On the basis of initial mapping studies, which were done with surface stimulation, the motor cortex was described as having a topographic organization that parallels that of the somatosensory cortex. The face, body, and upper limb were represented on the lateral surface with the face located inferiorly, near the lateral fissure, the torso most superiorly, and the lower extremity mostly on the medial aspect of the hemisphere. This somatotopic organization is often represented as a figurine or in a graphic form called a motor homunculus (
Fig. 9.15B ). The distortion of the various body parts in the homunculus indicates approximately how much of the cortex is devoted to their motor control. This simple homunculus was likened to a piano keyboard and fit well with traditional conceptions of the motor cortex being the final cortical stage and acting as a relay for sending motor commands to the spinal cord.
Beginning in the 1960s and 1970s, investigators in mapping studies began using microelectrodes inserted to the deep, or output, layers of the cortex to apply stimuli. With this technique, called intracortical microstimulation, much lower stimulus intensities could be used to evoke movements and thus allowed higher resolution mapping of the motor cortex, which revealed a much more complex topography than was previously imagined (see
CHAPTER 9 Organization of Motor Function
Fig. 9.15C ). Movement about each joint was found to be evoked by many noncontiguous columns throughout wide regions of the motor cortex. Thus cell columns related to movement about a particular joint are actually interspersed among columns that control movement about many other joints. In sum, although the motor cortex may have large subdivisions corresponding to a limb or the head, within each such area there is a complex intermingling of cell columns that control the muscles within that body part.
Such mixing of cell columns makes functional sense because most movement requires the coordinated action of muscles throughout a limb and most connectivity in the cortex is localized (i.e., axon collaterals that connect different cell columns are primarily confined to a 1to 3-mm region surrounding the column from which they originate). Thus when multiple cell columns that control movement about a joint are present and intermixed with columns controlling movement about other joints, multijoint movement can be generated as a whole.
Although the topographic map of the motor cortex is in part anatomically determined by the topography of the corticospinal pathway, it is also a dynamic map. Axon collaterals link the different cell columns, so that activity in one column could potentially lead to movement about multiple joints. In fact, this can happen, but these intercolumnar connections are modulated by inhibitory interneurons that transmit or secrete gamma-aminobutyric acid (GABA). This was shown by locally blocking GABA in one region of the motor cortex and then stimulating the neighboring region. Before the block, stimuli evoked contractions of one set of muscles, but once inhibition was blocked, contractions were also evoked in muscles controlled by the region that was no longer inhibited (
Fig. 9.16 ). Functional connections between cell columns can be controlled on a millisecond time scale, and depending on their state, the motor cortex map can be radically changed. Longer term plastic changes are also known to occur; for example, the use (or disuse) of a body part can affect the size of its somatotopic representation.
The supplementary motor area (SMA) is located mainly on the medial surface of the hemisphere, just anterior to the primary motor cortex, and corresponds to the medial portion of Brodmann area 6 (see